[AArch64][SVE] Convert svdup(vec, SV_VL1, elm) to insertelement(vec, elm, 0)

By converting the SVE intrinsic to a normal LLVM insertelement we give
the code generator a better chance to remove transitions between GPRs
and VPRs

Co-authored-by: Paul Walker <paul.walker@arm.com>

Depends on D101302

Differential Revision: https://reviews.llvm.org/D101167